The First Successful Casing in Casing Cementing for Re-Fracturing Treatment in China: Case Study from China’s First Shale Gas Field

Abstract

The Fuling shale gas field in China was discovered in 2012 and is a quality high pressure natural shale gas reservoir in the Longmaxi formation. Since then, more than 500 wells have been drilled and are being produced. However, over the period there has been a reduction in production and therefore refracturing is needed to maintain production. A new casing-in-casing method of re-constructing a new wellbore inside the legacy wellbore for re-fracturing was introduced and successfully executed. The producing wellbore has a series of perforations along the casing of the horizontal section. Wellbore re-construction is required to isolate all these perforations and allow a plug-and-perf fracturing process in the new wellbore. It was planned to run a 3.5 inch casing into the existing 5.5 inch casing and cement it. A dependable cement barrier in this extremely tight annulus is required to carry on future re-frac operations. Computational fluid dynamics and stress modelling were performed to optimize the slurry density, rheology, mechanical properties and based on various iterations tailored ceramic centralizers were proposed to achieve zonal isolation objectives. The top of cement (TOC) in the annulus is required to be above the topmost planned perforation. The remaining 3.5 inch casing above the designed depth was disconnected and pulled out. A new 5.5 inch X 3.5 inch wellbore without any leaks to the existing perforation was constructed. The wellbore was reamed to bottom, and the losses were treated prior to cementing. Tailored ceramic centralizers were installed on the casing to achieve optimum stand off along with a low friction factor which helped casing to run to the bottom successfully. A low rheology slurry tailored for optimum mechanical properties to withstand the fracturing operation was pumped and the cement returns to designed depth were noted. Cement bond log showed excellent results and the stage fracturing operation was performed with no issues with wellbore integrity. A tailored slurry and centralizer design helped to achieve zonal isolation objectives in the low clearance casing-in-casing (CiC) cementing configuration. The critical wellbore re-construction objectives were achieved, and the well was re-fractured with substantial increase in production.